Oscillators having a plurality of clock phases are frequently used for scanning received data in receiver circuits. In the case of serial high-speed transmitter/receiver chips (transceivers), in particular so-called quadrature oscillators, which emit four scanning signals offset successively by 90° in each case, are used for this purpose. However, widely-used basic circuits of these quadrature oscillators have the disadvantage that, because of random processes when starting, the direction of rotation of the oscillators may be either anticlockwise or clockwise.
This problem of the different directions of rotation is represented in FIGS. 1A and 1B using the example of a quadrature oscillator. In FIG. 1A scanning signals P0, P1, P2 and P3 outputted by such a quadrature oscillator over time t are represented. The signals are offset successively to one another by 90° or a quarter of a period. The direction of rotation is indicated by an arrow R.
In FIG. 1B, by contrast, the scanning signals P0–P3 of a quadrature oscillator rotating anticlockwise are illustrated, as indicated by an arrow L. On closer inspection of FIGS. 1A and 1B it is noticeable that a reversal of the direction of rotation corresponds to a transposition of signals P1 and P3.
In principle, scanning of the data is possible with both directions of rotation. However, because a change of direction of rotation also causes a change of the corresponding data signals scanned with the scanning signals, in the example illustrated the scanned data signals belonging to the scanning signals P1 and P3 would also be transposed. This makes defined processing of the respective scanned data signals impossible.
It is therefore an object of the present invention to provide a method and an apparatus with which a defined data output is achieved even with the use of scanning signals which are phase-offset to one another in an unknown direction.